Method and an apparatus for determining the refraction

ABSTRACT

The invention relates to a method for determining the refraction of the human eye. The retinal image of an object is imaged via a lens system into an object space and into an image plane which is conjugated with respect to the retina. The spacing of the conjugated plane is analyzed to determine the refraction of the human eye. This analysis is carried out by means of a measuring object which is moved along a plane which is defined by rays of the beam of rays which pass through image points of the retinal image of a target object, such that the beam of rays is divided into two complementing partial beams containing an unevenly divided amount of light energy. The unevenly divided light energy is used to determine the distance of said conjugated plane.

The invention relates to a method and an apparatus for determining therefraction of the human eye, i.e., in short, the status of refraction ofthe human eye. The invention relates specifically to a method fordetermining the status of refraction depending on the time. Inaccordance with the invention an object in the object space is imaged bymeans of an optometer lense system, in short an optometer lense, ontothe retina of the eye; the retinal image of said object, in turn, isimaged back via said optometer lense into the object space into acertain image plane, said image plane being conjugated to the retina andbeing defined by the status of refraction of the eye and the refractingpower of the optometer lense; the distance of said image plane from saidoptometer lense being representative of the status of refraction, andwherein according to the invention for determining said distance thebeam of rays coming from said retinal image is analysed at locationshaving different distances with respect to said image plane.

Automatic methods for determining the refraction are known (see FIG. 1).According to said known methods the image plane conjugated with respectto the retina of the tested eye is detected. The methods known fromGerman Auslegeschrifts Nos. 29 37 891, 31 10 576, 31 02 450 provide forthe imaging of an object, which may have the form of a grating, a point,or a slit, by means of an optometer lense system onto the retina.

The retinal image is reflected back (as a reflected image) into theobject space by means of the optometer lense and a beam split mirror(beam splitter). Said reflected image is analysed in the object space bymeans of another target object. For the purposes of said analysis theluminous flux which is transmitted by said target object is determinedby a photo sensitive element. It is therefore necessary that bothobjects correspond to each other as far as shape and dimension areconcerned. So as to detect the conjugated image plane, the two objectsare moved together with the beam splitter along the optical axis, or,according to a different method, the refracting power of the optometerlense system is varied. As soon as the position of the conjugated planeis found, a maximum, or depending on the kind of arrangement used, aminimum, of the luminous flux is measured behind the target object bymeans of the photo sensitive element. The refracting power of the eye isnot supposed to vary during said movement. With such a method thevariation of the refracting power of the eye depending on time can notbe determined without difficulties.

German Auslegeschrift No. 22 62 886 provides for an improvement byperiodically moving the object system with high frequency for only asmall distance along the optical axis. The result is a periodic changeof the photo signal synchronous to said periodic movement. From thephase shift between the movement of the object system and the photosignal an electrical photo signal may be derived for a sufficiently highoscillation frequency, a photo signal which indicates into whichdirection the object system has to be moved so as to have the conjugatedplane being arranged in the center between the points of reversal of theperiodic movement. This method has the disadvantage that high mechanicaloscillatory frequencies are required, i.e. high values for theacceleration are necessary. Moreover, the determination of the phase isdifficult because of the low signal/noise-ratio of the photo signal.

The method of German Auslegeschrift No. 26 54 608 provides for a certainimprovement with regard to the mechanical oscillations. Said knownmethod replaces the continuous oscillation by a measurement only at twolocations. However, even with this method the signal necessary forcontrolling the movement of the object system along the optical axis hasto be derived indirectly from the phase relation between the physicalposition and the photo signal.

Another principal method for automatically determining the refraction,is the method of skiascopy, described below.

It is an object of the present invention to avoid the disadvantages ofthe prior art.

A specific object of the invention is to provide a method and anapparatus for determining the refraction such that a photo signal havinga low S/N-ratio is provided, a signal which allows fast determination ofthe conjugated plane in an easy and precise manner.

The above objects of the invention are solved by characterizing portionsof claims 1, 2 and 3. Preferred embodiments of the invention aredisclosed in the other claims.

Additional advantages and essential features of the invention may begathered from the following description of embodiments of the invention;in the drawing:

FIG. 1 is a representation of a prior art method for measuring therefraction;

FIG. 2 is a representation of the basic principle used in themeasurement method shown in FIG. 1 when detecting the conjugated plane;

FIG. 3 is a first embodiment of the invention;

FIG. 4 is a second embodiment of the invention;

FIG. 5 is a third embodiment of the invention;

FIG. 6 is a representation of the principle used for determining theconjugated plane in accordance with the method disclosed in FIG. 5;

FIG. 7 is a forth and particularly preferred embodiment of theinvention;

FIG. 8 shows the wave form of the photo signals obtained in anexperiment when the movable object system is moved in accordance withthe method disclosed in FIG. 7;

FIGS. 9a, 9b disclose a fifth and particularly preferred embodiment ofthe invention;

FIGS. 10a, 10b disclose a modification of the embodiment of FIGS. 9a,9b.

FIG. 1 discloses, as already mentioned, a prior art method fordetermining the refraction. According to said method an image plane 4and 4', respectively, an image plane which is conjugated with respect tothe retina 3 of an eye O and having a distance 9 from the optometerlense system 1 is detected. Said distance 9 is representative for thestatus of refraction of the eye O. To this end, an object 6 is imagedonto the retina 3, said object 6 may be in the form of a grating, apoint or a slit. An image of the retinal image 2 is reflected back intoan object space 5 by means of the optometer lense system 1 (optometerlense 1) and a beam splitter 7; said reflected image is analysed bymeans of a target object 6' located in said object space. For thatpurpose the luminous flux which passes the target object 6' is measured.

For this purpose both objects 6, 6' do have to correspond to each otheras far as form and dimension is concerned. So as to detect theconjugated image plane 4 and 4', respectively, the object system 19comprising the two objects 6, 6' and the beam splitter 7 is moved alongthe optical axis 18. It would also be possible to have the beam splittermaintain its position. Another possibility would be to change therefracting power of the optometer lense system 1. FIG. 2 explains thissituation. In said fig. the imaging beam of rays for one image point 61'(61 in FIG. 1 of the image of a point of the retinal image 2 is markedin the conjugated plane 4' (4). As soon as the target object 6' (whichis here for reasons of simplification a pin hole stop) is in front ofthe conjugated plane 4', e.g. in position 31', a portion of the beam ofrays 30 is removed from the target object 6', a portion whichcorresponds to the amount of defocussing; said portion increases withincreasing distance of the target object 6' from the conjugated plane4'. The same is similarly true for all positions 31" of the targetobject 6' behind the conjugated plane 4'. Only if the target object 6'is in the conjugated plane 4' (see 31 in FIG. 2) the entire lightengergy contained in the beam of rays 30 can pass through said targetobject 6' in an ideal situation. In such a situation a maximum, or inother embodiments a minimum, of the light flux A delivered from thephoto element 8 in FIG. 1 is measured behind the target object 6', saidmaximum (or minimum) being an indication for the position of theconjugated plane 4' and 4, respectively. The refracting power of the eyeO is not supposed to change during the movement along the optical axis18. With such a method the change of the refracting power of the eyedepending on time can not be determined without problems. Referencenumeral 17 refers in FIG. 1 to the optical axis of the eye.

In the following figures similar reference numerals, as were used inFIGS. 1 and 2, will be used for similar elements, unless otherwisestated.

FIG. 3 discloses an improvement of the invention relating to theelectrical evaluation of the photo signal. In accordance with thisimprovement measurements are carried out simultaneously before andbehind the conjugated plane 4. For this purpose one target object 6' isarranged before the conjugated plane 4', and a second target object 6"is arranged with a fixed distance with respect to 6' behind theconjugated plane 4". This arrangement may be realized for instance byproviding another beam splitter 7'. Behind each target object 6', 6"each one photo element 8, 8' is located. If the object system 19 ismoved along the optical axis 18 (see FIG. 2) the light flux of theimaging beams of rays 30 (path of beams of measurement), which istransmitted in accordance with FIG. 2 by the target objects 6' and 6",respectively, will steadily decrease with increasing distance from theappropriate conjugated planes 4' and 4". Inasmuch as the two targetobjects 6' and 6" are moved together maintaining a fixed distance fromeach other, both said target objects 6' and 6" cannot both be located atthe same time in the conjugated plane. The photo signals A and B willconsequently reach their maximum for different positions of the objectsystem. The photo element which belongs to the target object whichhappens to be closer to the conjugated plane will provide the largerphoto signal. Only in the case where the two target objects 6', 6" havethe same distance from the appropriate conjugated plane 4', 4", i.e. ina situation where the image of the retinal image on the two targetobjects 6', 6" is equally defocussed and the conjugated plane isarranged between said two objects, then the photo signals A and B are ofthe same size. In all other cases the difference A-B provides a photosignal for a controlled movement of the object system 19, comprising theelements 7, 7', 6, 6' 6"; 8, 8' in the direction towards the symmetricalposition with respect to the planes 4, 4', 4".

In addition to the image point 61' of FIGS. 1 and 2, already explained,another image point 61" is shown. The information concerning thedirection of movement is now no longer contained in a phase, butadvantageously directly in the amplitude of the photo signals A and B.However, the gain with respect to the S/N-ratio can not be fullyutilized, because the additional beam splitter 7' is necessary.

This disadvantage may be overcome (FIG. 4) if the object 6 and thetarget object 6' are provided with an extension in the direction of theoptical axis 18 such that two parallel partial objects 61, 62 and 61'and 62', respectively, are present. If now, as is examplified in FIG. 4,behind each partial object 61' and 62' of the target object 6' a photoelement 8, 8' is positioned, a photo element which measures the lightflux in the form of photo signals A and B passing through the partialobjects 61', 62', it is again possible to find in a determined mannerthe conjugated plane 4, 4' by means of the clearly directed differentialsignal (A-B)-similar to the embodiment of FIG. 3-by moving the objectsystem 19 along the optical axis 18. The conjugated plane will bereached when both photo elements 8, 8' provide equal photo signals A andB. Even though in this manner the photo signal is improved, theS/N-ratio is still not favorable for a continuous registration of thecondition of refraction.

A completely different method for automatically determining therefraction is the so-called cutting edge method (skiascopy); see forinstance German Auslegeschrifts Nos. 2315135, 2951897 and 3020804 whichrelate to the automatic refractometry. In accordance with this method itis not the sharpness of an image which is checked, but, in the mostgeneral sense, the width of a beam of rays is determined, a width whichhas to be minimal in the conjugated plane. In accordance with suchmethods, the width of the imaging beam of rays is determined timewisesuccessively at different distances with respect to an optometer lenseperpendicular to the optical axis; starting from the differential signala phase depending value is calculated, a value by means of which aposition with the smallest or the same diameter of beam is located. Thisposition is reached when the conjugated plane is arranged in the middlebetween the two positions of measurement.

The embodiment shown in FIG. 5, in a certain sense, makes use of theskiascopic method. In contrast to the method of the prior art, however,the cutting edges are not moved perpendicular to the direction of rays,but they are moved in the direction of the imaging rays (imaging path ofrays). In the explanatory FIG. 6 the imaging beam of rays 30 of oneimage point of the image of a point of the retinal image 2 in theconjugated plane 4' of FIG. 2 is split by means of a plane 32', a planewhich is defined by the imaging beams 32 extending through the imagepoint 61', into two partial beams of rays 30a (hatched) and 30b,respectively, which are above and below said plane 32'. In the twodimensional representation the plane 32' coincides with the beam 32. Asis shown in FIG. 6 the beam of rays 30a (30b) has a different light fluxbefore and after the image plane. The closer the plane 32' and the ray32, respectively, come to the marginal ray 33 and 34, respectively, ofthe beam of rays 30, the larger is said difference. The change of thelight flux of at least one of said partial beams of rays 30a and 30b,respectively, at the location of the conjugated plane 4' is used forfinding the conjugated plane 4'. For this purpose the flux of light ofat least one of said partial beams of rays 30a, 30b has to be measuredalong the beam 32, as is shown for example by positions 31' and 31". Itis possible to arrange for that purpose, as is shown for the embodimentof FIG. 5, the optical axis 17 of the eye in a parallel offset mannerwith respect to the axis 18 of the apparatus. At least at onebright/dark transition 61' and a dark/bright transition 62' the flux oflight, respectively, which can pass the target object 6' is measured bymeans of two photo elements 8, 8' in the form of photo signals A and B.An improvement of the electrical S/N-ratios is here obtained because themeasurement is carried out not timewise successively, but simultaneouslywith the consequence that the information is not contained in the phasebut in the amplitude of the photo signals A and B. Measurements of thedifferential signal A-B of the photo elements 8, 8' when moving theobject system 19 along the optical axis 18 result for this method in adifferential signal which can be well processed so as to find theconjugated plane 4 and 4', respectively. However, the local change ofthe differential signal, i.e. the sensitivity, is small in theneighborhood of the conjugated plane.

An additional improvement may be obtained when using the method of theevaluation of sharpness in accordance with FIG. 4 together with the lastmentioned method disclosed in FIG. 5; see FIG. 7. By providing aparallel movement of the optical axes 17 and 18 the conditions of FIG. 5are realized, and by providing the oblique position of object 6 theconditions of FIG. 4 are simultaneously realized. FIG. 8 shows theappropriate form of the photo signals A and B of the two photo elements8, 8'. The photo element 8 supplies primarily a photo signal A if theappropriate edge 61' of the target object is in front of the conjugatedplane 4', and, in contrast thereto, the photo element 8' supplies aphoto signal B if it is located behind the conjugated plane 4'. Theregion of overlap C (see FIG. 8) of the two photo signals, which causesa steeper zero-crossing of the differential D of the two photo signals Aand B, is caused by the simultaneous realisation of the additionalcriteria of sharpness in accordance with FIG. 4.

An additional improvement of the S/N-ratio can be obtained for themethod of FIG. 4 as well as for the method of FIGS. 3, 4 and 5 if thebeam splitter 7 is deleted and the object 6 itself is provided in theform of a mirror, as is shown for the two embodiments (FIGS. 9 and 10)discussed below; object 6 and target object 6' are reduced to a singleobject. This solves all problems of adjustment regarding the object 6and the target object 6', adjustments which otherwise occur due to thehigh requirements relating to the geometric similarity and the correctoptical positioning of the objects 6 and 6'.

Each of the FIGS. 9 and 10 disclose particularly preferred embodimentsof the invention, embodiments for which an improvement of the S/N-ratiois possible and which allow the automatic measurement of the status ofrefraction and the continuous documentation by means of printer and/orplotter, a measurement which is carried out by means of light reflectedby the retina while at the same time the energetic stress of the eye isreduced. The two embodiments differ from each other in so far, as theimaging system of FIGS. 9a, 10a and the measuring system of FIGS. 9b,10b are exchanged. Consequently, the description can be restricted to adescription of FIG. 9, and said description can be applied also to FIG.10.

The optical arrangement of the method comprises an imaging system (inFIGS. 9a and 10a: 1, 6, 61, 62, 10, 11) and a measurement system (inFIGS. 9b, 10b: 1, 6, 61, 62, 12, 13, 8, 8') which is separately depictedfor reasons of clarity; said systems are the same with respect to thetwo essential elements, i.e. optometer lense 1 and object 6. For reasonsof simplification the parallel offset arrangement of the optical axes 17and 18, as shown in FIG. 5, was deleted. It is important that in theimaging system as well as in the measurement system the same object isused. For carrying out the measurement only the optometer lensemaintains a fixed distance with respect to the eye, while the otherelements 19, having a frame around them, are moved in common in thedirection of the optical axis, or, alternatively, the image side focallength of the optometer lense is varied. So as to determine therefraction of the eye in different principal sections, the arrangementmay be rotated about the optical axis of the system, or, when the methodof FIG. 7 is realized, the rotation may be provided about an axisparallel thereto. The frontal principal plane 14 of the eye coincideswith the frontal focal plane of the optometer lense 1.

The imaging path of rays (FIG. 9a): In the refractometer according tothe invention an object 6 is obliquely arranged with respect to theoptical axis 18 having two partial objects in the form of parallelbright/dark 61 and dark/bright edges 62, edges which are illuminatedfrom the front by means of a high frequency modulated light having a lowaperture. The illumination may be carried out by means of a lightemitting diode 11 which for example emits light having a wave length of820 nm and is located in the focal point of a condensor 10. The use oflight having a low aperture provides by means of the optometer lense 1 asmall artificial entrance pupil of the imaging beam of rays in thefrontal principal plane 14 of the eye. The consequence is that also theaperture in the eye is small and the object 6 has, as seen from thesubject the form of a dark red illuminated slit, which, in praxis issharply seen over a wide range of distances of vision. The mechanism ofaccomodation is, consequently, not disturbed by the imaging system. Soas to minimize objectionable reflexes the optometer lense 1 may beinclined with respect to the optical axis 18, or else for the imaging ofthe object onto the retina polarized light might be used which isdepolarized on the retina, said light will maintain to a large degreeits direction of polarisation when being reflected at reflectingsurfaces. Said reflections may be separated from the reflected light ofthe retina by means of a crossed analyser arranged in the beam ofmeasurement.

Beam of measurement (FIG. 9b): The retinal image 2 of the object havingthe parallel edges 61, 62 forming partial objects is reimaged onto theobject 6 which creates the retinal image 2, said reimaging beingeffected by means of the light reflected by the retina 3 via the opticalarrangement of the imaging system and the optometer lense 1. At theedges a portion of the light will not return to the path of the imagingbeam, said portion of the light corresponds to the defocussing and foroptical axes being parallel offset, to the width of the beam of rays;said portion of the light will, instead, pass into the path of themeasurement beam 12, 13, 8, 8' (hatched beam of rays). For the detectionof the rejected defocussed portion of the high frequency modulatedenergy reflected at the retina 3 each of said two object edges (partialobjects 61,62) is imaged by means of a lense 13 via a field lense 12onto one of said two photo elements 8, 8'. Inasmuch as the frontalprincipal plane 14 of the eye (approximately the plane of the pupil)falls together with the focal plane of the optometer lense 1, andbecause the lense 13 is arranged in the backward focal plane of thelense 12, the two lenses 1 and 12 form a telecentric system, a systemwhich independently from the position of the movable object arrangement19 images the frontal principal plane 14 of the eye onto the lense 13.As a consequence thereof it is possible to possibly remove in front oflense 13 undesired reflexes of the imaging system in the frontal eyemedia. This will not be necessary if optical eye axis and the axis ofthe apparatus are parallel offset. In case that polarized light is usedit is possible to arrange between object 6 and photo elements 8, 8' acrossed analyser which will filter out the light occurring at the lensesurfaces having mirror characteristics.

Determination of the conjugated image plane: In case that for a fixeddistance of vision of the eye (accomodation) the movable apparatus 19,shown in the drawing within an enclosure, is moved along the opticalaxis 18, each of the proportions of the reflected light separated ateach edge (partial objects 61,62) of the object 6, proportions whichlead to the differential signal D of the photo elements 8, 8', willfirst decrease starting from the optometer lense 1 with increasingdistance from said optometer lense 1 corresponding to the position ofplane 4 conjugated with respect to retina 3, the zero crossing being inplane 4 and thereupon another decrease is following (see in FIG. 8 thedifference D). The differential signal D will exactly pass through thevalue zero when the two object edges 61, 62 are symmetrically equallydistantly located from the conjugated plane 4. Therefore, a zero methodbeing relatively undisturbable is being provided. It is the object ofthe method to position the movable arrangement 19 such that the twoobject locations 61 and 62 are symmetrically located with respect toplane 4. For this purpose the movable part has to be positioned withrespect to the lense 1 in a close loop control circuit such that thedifferential signal becomes zero (see FIG. 8).

The electronic photo signal processing (FIGS. 9b, 10b): So as to improvethe S/N-ratio and for filtering out room light, the light of measurement(for example the light of a LED) is high frequency modulated. Thedifferential signal d of the photo elements 8, 8' is amplified in aphase locked manner and amplified (lock-in amplifier 20). Thedifferential signal D thus amplified is fed into aWindow-Schmitt-trigger 21 having two trigger thresholds T1 and T2 (seeFIG. 8) which can be symmetrically adjusted with respect to zero volt.Said trigger supplies at its output signals for no movement, movementstowards the left and movements towards the right of the motor drive 22of the movable apparatus 19. In case that the differential signal D isbetween the two trigger thresholds T1 and T2, the position remainsunchanged. In case that the two thresholds T1, T2 are exceeded orunderceeded, the movable apparatus 19 is moved along the optical axis 18so long until the differential signal D is between said two thresholdsT1, T2. The light source 11 is controlled by means of a functiongenerator 24 which simultaneously controls via a phase shifter 23 thelock-in amplifier 20. The distance 5 (see FIG. 7) from the optometerlense 1 is the measurement value of the refraction value of the eye, avalue which is obtained for the sharp fixation of an object of vision ina predetermined distance from the eye. The determination of thisdistance 5 can for instance be determined by using a step motorconsidering the number of steps, or by means of a coupled potentiometer.

Visible target: For the determination of the condition of refractionvisible targets have to be offered to the subject; said visible targetshave to be offered to the subject; said visible targets are necessaryfor the fixation of the direction of vision and for the adjustment ofdistance of the eye for the aim of measurement in question (i.e.refraction, accomodation); said visible targets are adapted to stimulatethe accomodation. Said targets are projected between eye O and optometerlense 1 by means of an achromatic or, even better, a dichroitic beamsplitter mirror 15.

Recording of measured values: The values of refraction are supplied atthe output of the electronic means as analog or digital voltage values.Said values are, by means of ordinary printers, recorded, printed andfurther processed. By rotating the arrangement 1, 19 about the opticalaxis of the eye values of refraction for the different main sections maybe obtained when linear objects 6 (slit or grid) are used, values fromwhich the astigmatism with its direction of axis may be derived. Thiscan be done in the form of ordinary characteristics which arenumerically printed out, or, because of the good time resolution of themethod of the invention the output may be provided in graphic form, aform which allows an interpretation with respect to the regularity of anastigmatism in addition to the information given by the characteristics.In view of the good time resolution of the method of the invention theadjustment of the eye to distance depending on the time can be recordedparallel to the dependence on the distance of the visual object(refraction - time - characteristics) and it is possible to determineaberrations of the system of accomodation prior to their showing withmeasurements of static equilibrium adjustments without taking intoconsideration the time.

I claim:
 1. A method for determining the time dependent refraction ofthe human eye comprising the steps of:(a) imaging a target object havingan extension in depth in the direction of the optical axis in an objectspace onto the retina of the eye with an optometer lense system suchthat the retinal image of said object is re-imaged via said lense systeminto the object space and into an image plane which is conjugated withrespect to the retina and depends on the refraction of the eye and theindex of refraction of the lense system, the distance of said conjugatedimage plane from said lense system being a measure of refraction, (b)separating the beam of rays from the target object from the beam of raysfrom the retinal image by means of said target object comprising atleast two parial portions, said partial portions being edges of thetarget object and extending parallel to each other, and (c)simultaneously analyzing the beam of rays from the retinal image at atleast two different distances from the conjugated image plane by meansof said target object.
 2. The method of claim 1 wherein said partialportions are illuminated by means of a light source and a condensor in areflecting manner.
 3. The method of claim 1 wherein said partialportions are illuminated by means of a light source and a condensor in atransmitting manner.
 4. The method of claim 1 wherein said objectcomprises a mirror.
 5. The method of claim 1 wherein the beam of raysfrom the retinal image is analyzed so as to determine said distance ofthe conjugated plane by means of a measuring object, wherein saidmeasuring object is guided along at least one plane through said beam ofrays from said retinal image, said plane being defined by rays of thebeam of rays which pass through image points of the retinal image of thetarget object, and divides the beam of rays into two complementingpartial beams such that the light energy of the beam of rays is unevenlydivided between said complementing partial beams, causing unevenlydivided light energy in said partial beams, said unevenly divided lightenergy being used to determine said distance of said conjugated plane.6. The method of claim 5 wherein the target object and the measuringobject each comprise two edges, and wherein further the measuring objectis guided with each edge along a plane such that partial beams havingoppositely distributed light energy are measured.
 7. The method of claim5 wherein the measuring object is moved along the optical axis until thedifference between the light energies of the partial bundles havingoppositely distributed light energy is zero.
 8. The method of claim 5wherein the measuring object comprises at least one bright/dark anddark/bright edge which are used separately for carrying out themeasurement.
 9. A method for determining the time-dependent refractionof the human eye comprising the steps of:(a) imaging a target object inan object space onto the retina of an eye with an optometer lense systemsuch that the retinal image of said object is re-imaged via said lensesystem into the object space and into an image plane which is conjugatedwith respect to the retina and depends on the refraction of the eye andthe index of refraction of the lense system, the distance of saidconjugated image plane from said lense system being a measure ofrefraction, (b) separating the beam of rays from the target object fromthe beam of rays from the retinal image, and (c) analyzing the beam ofrays from the retinal image so as to determine said distance of theconjugated plane by means of a measuring object wherein said measuringobject is guided along at least one plane through said beam of rays fromsaid retinal image, said plane being defined by rays of the beam of rayswhich pass through image points of the retinal image of the targetobject, and divides the beam of rays into two complementing partialbeams such that the light energy of the beam of rays is unevenly dividedbetween said two complementing partial beams, causing unevenly dividedlight energy in said partial beams, said unevenly divided light energybeing used to determine said distance of said conjugated plane.
 10. Themethod of claim 9 wherein said plane is placed such that the partialbeam located on one side of said plane contains a negligibly smallportion of said light energy.
 11. The method of claim 9 wherein theoptical axis of the beam of rays from the target object and of the axisof the eye are parallel and offset to each other.
 12. The method ofclaim 9 wherein the edges of the target object and of the measuringobject are offset in longitudinal direction with respect to theappropriate optical axes.
 13. The method of claim 8 wherein saidmeasuring object comprises at least one mirror.
 14. The method of claim8 wherein the target object and measuring object are the same.
 15. Themethod of claim 8 wherein the measuring object comprises light sensitiveelements.
 16. The method of claim 9 wherein the edges of said measuringobject are parallel to each other.
 17. The method of claim 9 wherein themeasuring object cooperates with light sensitive elements, said lightsensitive elements providing output signals A, B by means of which adifferential signal (A-B) is formed.
 18. The method of claim 17 whereinthe differential signal A-B is a control signal for automaticallydetermining the image plane.
 19. The method of claim 18 wherein thedifferential signal A-B is amplified free of noise by means of a lock-inamplifier.
 20. The method of claim 9 wherein the system for carrying outthe measurement is mechanically or optically rotated about the opticalaxis of said measuring system or about an axis parallel thereto.
 21. Themethod of claim 9 wherein the imaging of the object onto the retinaoccurs by means of high-frequency modulated light.
 22. The method ofclaim 9 wherein for the imaging of the object infra-red light is used.23. The method of claim 9 wherein the aperture of the imaging system isselected to be small.